Minimising the bubble size through fluidic control of formation at a submerged orifice: The role of oscillatory inflow

Abstract

Bubble aeration has been widely used in water and wastewater treatment; however, it is associated with low gas utilization rate and high energy consumption. This study introduces a novel aeration method that reduces bubble size by modulating oscillating airflow through the orifice, thereby enhancing gaseous exchange rates. A dynamic model has been developed to simulate the bubble formation process under various oscillatory gas supply modes, elucidating the mechanisms by which oscillating airflow regulates bubble size. The key results identify frequency and amplitude of the oscillatory gas supply as critical factors influencing bubble formation. Specifically, increasing the oscillation frequency changes the direction of the inertial force, while greater oscillation amplitude enhances the gas momentum force. The oscillatory airflow significantly increases the upward force and weakens the dependence of the bubble detachment on the buoyant force, which leads to bubbles detached at an earlier stage. The maximum reduction rate of bubble size at 1 mm orifice is 74.5 %. It is worth noting that in continuous bubble formation under oscillatory gas supply, an increase in oscillation frequency results in a reduction of the average bubble diameter, while an increase in amplitude leads to a larger number of bubbles being produced. These results highlight the effectiveness of high-frequency oscillation gas supply in generating a larger number of smaller bubbles for mass transfer applications. The insights derived from this study contribute to a deeper understanding of bubble dynamics under oscillatory gas supply and offer practitioners with a new aeration mode choice aiming to improve the efficiency of bubble aeration.